Scanners using scanning or flying spots have found wide application in high resolution printing, document scanning, bar code scanning, and inspection, especially of continuous web materials. It is desirable at times to modify the size and shape of the scanning spot or its scan rate, or both, to accommodate special scanning situations. To do this, the scanner must monitor and measure the dimensions of the scanning spot, particularly its width in the scan direction and its length in the direction of the material movement. These measurements must occur while the equipment is in operation in order to provide feedback for continuous adjustment of the spot size.
Methods and equipment are readily available for measuring the dimensions of stationary spots, but the measurement of a scanning spot of light is much more complicated and obscure. Conventional spot width measurement techniques have been based on temporal measurements for inferring spot width, which require calculations affected by the velocity of the scanning spot. Similarly, conventional spot length measurement techniques have required arduous amplitude measurements complicated by scan-to-scan positioning error. Since both of these known methods require manual calculation or intervention during the measurement, neither of them lends itself to automatic feedback techniques for adjustment and control of the size and shape of the scanning spot.